This invention relates to a method for operating a heat-producing plant of the type which comprises a chamber for burning chlorine-containing fuels and a channel connected with said chamber, in which a plurality of consecutive heat-transferring devices are located, through which fluids, such as steam, water or air may pass with the purpose of being heated by hot flue gas which flows through the channel in the direction from the combustion chamber towards an outlet, the flue gas being conditioned by addition of a sulphur-containing additive, which is injected into the flue-gas flow in a channel section located, on one hand downstream a combustion zone, in which substantially all fuel is burnt, and, on the other hand, upstream and at a distance from that heat-transferring device positioned downstream the combustion zone being the first to be hit by the flue gas, and which additive, after the entrance thereof in the flue-gas flow has the purpose of, during the way thereof to said first heat-transferring device, sulphating above all gaseous alkaline chlorides comprised in the flue gas, with the aim of reducing chlorine-induced corrosion on the heat-transferring devices.
By U.S. Pat. No. 4,043,768 is previously known how to add a sulphur-containing additive to the type of flue gas that is obtained by burning coal of a varying sulphur content. As preferred additives, finely powdered ammonium sulphate or ammonium bisulphate are mentioned, which should be inserted into the flue gas when the same has a temperature within the range of 590-900xc2x0 C. The primary object of the addition of the sulphur-containing additive is to improve the efficiency of an electrostatic dust separator. Thus, what forms the basis of the technique being presented in U.S. Pat. No. 4,043,768 is the understanding that the dust separator works most efficiently in burning coal of a relatively high sulphur content (3-5% sulphur) as a consequence of said fuel providing a resistivity of the flue-gas particles of 108-1010 ohm cm, while coal of a low sulphur content (1%) provides a considerably higher resistivity (1013 ohm cm) of the particles, and thereby a considerably lower precipitation capacity of dust separators. By adding suitable amounts of a sulphur-containing additive, the resistivity of the flue-gas particles can be optimised at burning of coal of a low sulphur content; all with the purpose of attaining a good dust separation. In the described plant, the dust separator is located immediately upstream the flue-gas outlet or chimney, but down stream a plurality of heat-transferring devices, such as a secondary superheater, a reheating superheater, a so-called xe2x80x9cball-roomxe2x80x9d, a primary superheater, an economizer and an air preheater. In the preferred embodiment, which is described in U.S. Pat. No. 4,043,768, the sulphur-containing additive is inserted at a position upstream the dust separator, but downstream a number of heat-transferring devices. More precisely, it is preferred that the additive is injected into a section located between the inlet to said xe2x80x9cball-roomxe2x80x9d and the primary superheater, where the temperature is within the range of 900-550xc2x0 C.
Solid fuels in the form of bio and waste fuels are becoming an increasingly established energy source for combined power and heat production, among other things as a consequence of such fuels being long-term available and providing an energy-efficient combustion. The category of bio fuels includes, among other things, wood chips, bark, straw, sawdust, black liquor and the like, while waste fuels may, for instance, contain sorted domestic waste, industrial waste, demolition wood, sludge. Principally, waste fuels should be understood as such materials that already have been used for another purpose, while bio fuels are such plant materials that are utilised from the nature without other purposes than producing energy.
Since bio and waste fuels have been put into operation on a large scale, such fuels have, however, in several aspects proven considerably more difficult to burn than coal. Among other things, this is due to the fact that the ash of the bio and waste fuels has a different composition and different melting characteristics than the ash of coal. One of the most expensive problems is corrosion and ash deposits on the super heater tubes and other parts being comprised in the heat-transferring devices of the plants. Consequently, severe high-temperature corrosion has been detected in a large number of heat-producing plants after a few years of operating time with 100% bio fuels. At wood-fuel burning, the corrosion begins at steam temperatures in the superheater of approx. 480xc2x0 C., and increases along with the temperature up to approx. 500-600xc2x0 C. This equals the steam temperature of the hottest superheaters in modern combined power and heat plants. There is a trend in the industry to mix in demolition wood and sorted waste fractions in the fuel mixture, which may further accentuate the above-mentioned problems and extend the corrosion also to heat-transferring surfaces of lower material temperature. By those skilled in the art, chlorine is regarded as the principal corrosion accelerator. It is assumed that the chlorine is transported to the surface of the superheater tubes in the form of gaseous-phase alkaline chloride (at wood fuel mainly potassium chloride), alternatively as very small aerosols of alkaline chloride having condensed just upstream the superheater. On the tube surface, reactions between the alkaline chlorides and, for instance, ferrous oxide take place while forming free chlorine, which in this form is highly corrosive. The exact mechanism for the corrosion is not entirely clarified, but it is beyond all doubt that chlorine plays a central role.
By those skilled in the art, the relation of sulphur/chlorine or the S/Cl ratio of the fuel has been proposed as a parameter indicating the risk of chlorine-induced high-temperature corrosion. It is previously known that addition of sulphur to the fuel reduces chlorine-induced corrosion at waste burning. It has been assumed that an S/Cl ratio higher than 4 would be sufficient in order for the superheater corrosion to become acceptably low. Wood fuels have, however, an S/Cl ratio within the range of 1-2, and unsorted domestic waste approx. 0,2-1.
Also fuels of a relatively high S/Cl ratio may give rise to problems related to alkaline chlorides, viz. if they are burnt under conditions which mean that a major share of the sulphur is bound in the form of sulphate compounds already in the burning process. Such conditions prevail, for instance, at the burning of coal in a fluidized bed at addition of lime-stone, dolomite or other compounds that form basic oxides in the combustion chamber with the purpose of decreasing the emission of SO2. Especially at a relatively high dosage or the presence of the sulphur-binding additive, in order to meet stricter environmental requirements, chlorine corrosion has occurred on superheater surfaces situated downstream the combustion zone.
The hypothesis concerning the ability of sulphur to decrease the chlorine content of the superheater deposits is that the alkaline chlorides of the flue gas is converted into alkaline sulphate before they reach the superheater according to the following sum formula:
2KCl+SO2+xc2xdO2+H2OK2SO4+2HClxe2x80x83xe2x80x83(1) 
the equilibrium of which is displaced to the right at flue-gas temperatures below approx. 800xc2x0 C.
Test measurements have shown that a fairly high concentration of SO2 is required in the flue gas in order for KCl to be significantly reduced in flue-gas temperatures at about 700-900xc2x0 C., which equals the temperature range within which the superheaters in the most common types of boilers operate. For instance, measurements at burning wood chips have shown that an SO2 content corresponding to approx. 50-150 mg S/per MegaJoule (MJ) fuel is required in the flue gas in order to achieve a 50% reduction of the potassium-chloride content in the flue gas ahead the superheaters. This equals an S/Cl mole ratio in the flue gas of approx. 3-10. The increase of the sulphur-dioxide content in the flue gas entails negative consequences in the form of increased SO2 emissions from the plant (problems with emission regulations and environmental permits) and an increased risk of low-temperature corrosion. The reason for such a large SO2 redundancy being required is that the reaction (1) is kinetically limited at the flue-gas temperature in question, and has therefore no time to reach equilibrium before the chloride encounters the superheater.
The reaction (1) is an overall reaction including a plurality of substeps. An important substep is considered to be:
SO2+O(+M)SO3(+M)xe2x80x83xe2x80x83(2) 
where M is a catalyst for the oxidation. The sulphur trioxide formed will further react with the alkaline chlorides according to the formula:
2KCl+SO3+H2OK2SO4+2HClxe2x80x83xe2x80x83(3)
M may, for instance, be ferrous oxide. The equilibrium of the reaction (2) moves to the right at decreasing temperature. At temperatures of about 700xc2x0 C., the content of SO2 and SO3, respectively, is equally large at equilibrium (depending on the sulphur content and oxygen content). In absence of the catalyst M, reaction (2) is slow at temperatures below 900xc2x0 C. In flue gas, normally the concentration of M is low in relation to the gaseous phase reactants oxygen and sulphur dioxide. As a consequence, just a minor part of the sulphur dioxide has time to oxidise to sulphur trioxide before the flue gases leave the plant. This has been verified by measurements of the SO3 content in flue gas at approx. 150xc2x0 C., which indicates that only approx. 1-3% of the total sulphur-oxide content in gaseous phase is present as SO3.
Addition of sulphur at the burning may be effected in different ways. The most commonly used method is co-burning a fuel of a low S/Cl ratio (e.g. bio fuel) with a fuel of a higher sulphur content, for instance commonly used coal types or peat. However, measurements have shown that the share of coal or peat has to be relatively large (20-50%) in order to achieve a pronounced reduction of the flow of alkaline chlorides to the superheater. Such a large admixture of fossil fuel is not always consistent with the external and internal environmental requirements of the energy producer, and usually neither economically advantageous. Furthermore, many combustion plants lack fuel-handling systems adapted to handle a plurality of fuel flows. In connection with future economical control means for reduction of CO2 emission, co-burning with a large share of coal can be expected to become clearly unprofitable in comparison with pure bio- or waste-fuel combustion. Furthermore, bio and waste fuels, as well as most fuels rich in sulphur, are inhomogeneous in respect of, among other things, the sulphur content. This implies that different mixture ratios of the fuels are required at different times in order to maintain a sufficiently high S/Cl ratio, without the sulphur emission becoming high. Since the delays and buffer volumes of the fuel handling systems generally are considerable (a representative feed hopper may house fuel for approx. 1 hour of operation), it is evident that, in terms of control engineering, it will become very difficult to maintain optimum fuel ratios at varying sulphur and chlorine content in the fuel flows.
Another theoretically feasible method to supply sulphur is dosing elementary sulphur in the form of granule or powder directly into the fuel flow immediately before the fuel is fed into the boiler.
Irrespective of method of supplying sulphur to the fuel before the combustion according to the above, the emission of SO2 will increase considerably.
The present invention aims generally at obviating, or at least considerably abating, the problems that are associated with chlorine-induced corrosion on the superheaters or heat-transferring devices being comprised in heat-producing plants of the type in question. Therefore, a primary object of the invention in a first aspect is to provide an operation method, according to which the content of alkaline chlorides in the flue gas is reduced to a minimum before the flue gas hits the first one of the various heat-transferring devices, which are located in the flue-gas channel between the combustion chamber and the outlet of the plant, without giving rise to significantly increased SO2 emissions to the environment because of this. An additional object is to achieve the desired reduction of the alkaline-chloride content of the flue gas by means of sulphur quantities that are considerably lower than the quantities required in the sulphation methods described above.
According to the invention, at least the primary object is attained by the features defined in the characterizing clause of claim 1. Preferred embodiments of the method according to the invention are furthermore defined in the dependent claims.
Further Elucidation of Prior Art
By DE 19849022 is previously known, according to the way initially mentioned, how to dose a sulphur-containing chemical into the flue gas downstream a combustion zone and upstream a set of heat exchangers, with the purpose of reducing chlorine-induced corrosion on the heat exchangers. The originator of the method according to this patent publication has, however, not realised the benefits of primarily adding SO3, rather than SO2. Thus, the dosing of additive in the known method is adjusted on the basis of measured SO2 content in the flue-gas channel, and when the SO2 content is below a certain limit value, the dosing is started so that the SO2 content is increased above the limit value. As the only example of a sulphating additive, MgSO4 is mentioned, which at injection into the flue gas forms SO2, all measurements and adjustments of the flue gas taking place on the basis of SO2.
Contrary to the method according to DE 19849022, the present invention is based on the idea of primarily adding SO3 solely, whichxe2x80x94without slow oxidation of SO2xe2x80x94reacts very fast and efficiently with present alkaline chlorides while forming alkaline sulphate, which is considerably more harmless in respects of corrosion. Thereby, no surplus of SO2 is required to achieve a sufficient sulphating reaction. In other words, unnecessary SO2 emissions are avoided. Furthermore, according to the invention the flow of SO3 may quickly be adjusted to an altered boiler load and/or flue-gas composition so that an optimum S/Cl ratio is continuously maintained in the flue gas upstream all the superheaters or heat producing devices under varying burning conditions.
The method according to the invention is based on the idea of injecting an alkaline chloride-reducing substance or additive into the plant in an area or channel section which is located downstream the combustion zone in which substantially all fuel is burnt ( greater than 90% should be burnt in said zone), as well as upstream and at a distance from that heat-transferring device being the first to be hit by the flue gas. Contrary to this, in U.S. Pat. No. 4,043,768, supply of a sulphur-containing additive is recommended at a point located downstream a plurality of the heat-transferring devices comprised in the plant, which for this simple reason are subjected to chloric-induced corrosion. Furthermore, the additive should, in connection with the entry thereof in the flue-gas flow, in all essentials consist of a reactive sulphur trioxide (SO3), which is added in a quantity of at least 20 mg S/m3n (normal cubic meter) flue gas. The flue-gas temperature is usually within the range of 600-1200xc2x0 C. in the section in question of the combustion plant. The substance that is injected may be of varying forms, but is henceforth generally denominated xe2x80x9cadditivexe2x80x9d. Characteristic of the additive should, however, be that it generates SO3 at heating to the temperatures being at hand in the dosing area. According to the invention, for this purpose a compound is selected from the group: ammonium sulphate ((NH4)2SO4), ammonium bisulphate ((NH4)HSO4) and ferrous sulphate (FeSO4). Said salts are easily soluble in water and may advantageously be injected as an aqueous solution, e.g. an aqueous solution of 25% (percentage by weight) of an ammonium sulphate. A feasible liquid additive consists of sulphuric acid (H2SO4) of varying concentration.
By the injection taking place at a distance from the first heat-transferring device, the reactive sulphur trioxide has during the way thereof to said device time to sulphate gaseous alkaline chlorides in particular, so the same are reduced to a minimum before the flue-gas flow reaches said first heat-transferring device.
The liquid additives are suitably dosed by means of nozzles (see xe2x80x9cDosing Equipmentxe2x80x9d below). It is also feasible to inject SO3-forming additives of solid phase (particular form) in the boiler by means of, for instance, an air or flue-gas flow. This can suitably be realised by using a lance for the dosing of additive (see xe2x80x9cDosing Equipmentxe2x80x9d below). An additional possibility to achieve the desired effect according to the invention is to generate SO3 gas outside the proper boiler and lead the SO3-enriched gas into the flue gas in the specified section of the flue-gas channel. This may be achieved, for instance, by heating/boiling of sulphuric acid. Another method is to initially generate SO2, for instance by burning sulphur in furnace, and then lead the SO2 gas over a vanadium catalyst, where it is converted to SO3. Whether the heating of the SO3-generating substance takes place outside the boiler, inside the flue gas, or inside some kind of lance located in the flue-gas channel, is of incidental importance for the carrying out of the invention.
Advantages of the Invention
An advantage of injecting substances which directly form SO3, without taking the way round the slow oxidation of SO2, is the fact that SO3 reacts very fast and efficiently with the present alkaline chlorides while forming alkaline sulphate, which is considerably more harmless in respects of corrosion. Thereby, no surplus of SO2 is required to achieve the sulphating reaction, and thereby unnecessary SO2 emissions are avoided. Furthermore, the sulphur consumption may be limited, partly thanks to the more efficient reaction path mentioned above, partly by the adding of SO3 being made behind the proper combustion zone, where the share of sulphur which is bound to the basic oxides of the fuel ash or to de-sulphating products such as limestone or dolomite is minimal. The flow of the SO3 additive according to the invention may be quickly adjusted according to altered boiler load and/or flue-gas composition so that an optimum S/Cl ratio is continuously maintained in the flue gas upstream all the superheaters or heat-transferring devices under varying burning conditions, e.g. as a consequence of variations in the fuels.
Optimum Temperature Interval
The reason for the fact that no major quantity of SO3 is converted to SO2 at high temperatures, for instance close to 1000xc2x0 C., where the thermodynamic equilibrium is moved towards SO2, is that the kinetics of the reaction (2) according to the above is slow (which is valid in both directions). The rate of the reaction (2) increases with increased flue-gas temperature. When the gas temperature at the injection point rises above, for instance, 1000xc2x0 C., the conversion of SO3 to SO2 is, therefore, considerably increased, the efficiency of the method according to the invention decreasing. Therefore, the advantageous temperature interval of injection or supply of the SO3-generating substance is 600-1000xc2x0 C. At temperatures below 600xc2x0 C., the content of alkaline chlorides in a gaseous phase is very low because the dew point of the chloride gas is not exceeded. In that connection, the alkaline chloride condenses and forms very small (5-100 nm) aerosols in the flue gas. Also the sulphur trioxide sulphates a certain share of said alkaline chloride aerosols in those cases they are formed, but laboratory experiments have shown that the sulphating of alkaline chlorides takes place considerably faster in a gaseous phase than in a condensed phase. Therefore, it is advantageous to perform the dosing of the additive in an area where the major part of the alkaline chloride is in a gaseous form, i.e., at a certain distance downstream the first heat-transferring device, but downstream the proper combustion zone.
Selection and Adjustment, Respectively, of Optimum Quantity of Additive
The dosing of additive should in each application and operation method, respectively, be continuously adjusted to the flue-gas flow in question as well as the alkaline-chloride content in the flue gas. The flue-gas flow may be calculated on the basis of the producing effect of the boiler as well as the moisture content and elementary composition of the fuel, or be measured by methods known by those skilled in the art. The alkaline-chloride content in the flue gas may advantageously be measured with the technique that is described in the SE patent 9903656-B, or approximately calculated on the basis of information about the fuel""s chlorine content. Other factors that affect the choice of additive flow are, for instance, the sulphur content of the fuel, as well as the flue-gas temperature in the section of the flue-gas channel where the additive is introduced. The sulphur content of the fuel is initially oxidised to sulphur dioxide, and then said sulphur dioxide can sulphate alkaline chloride in the flue gas. However, only a small share of the fuel sulphur contributes to the sulphation of alkaline chlorides before the flue gases hit the superheater. Too low an additive flow results in a limited effect on the chlorine corrosion, while too high a flow implies an unnecessarily high additive consumption as well as an increased risk of corrosion and deposit formation on the low-temperature heat exchanger of the plant, for instance, the air preheater, which operates at material temperatures within the range of 80-200xc2x0 C.
It is not necessary to entirely eliminate the alkaline chloride in the flue gas in order to avoid chlorine corrosion.
A reduction of the concentration by about 50-80% is often sufficient in order to achieve a considerably milder corrosion. The required degree of reduction depends on fuel composition, operation conditions, superheater materials, steam temperature etc., and should be determined from case to case.
Theoretically, according to the reaction (3) above, 0,5 mole SO3 should be sufficient to sulphate 1 mole alkaline chloride. Practical measurements in combustion plants indicate, however, that an addition of additive equivalent to a mole ratio of SO3/[KCl+NaCl] within the range of 0,75-2,0 is required to obtain a chloride reduction of 75%. Here, the expression [KCl+NaCl] represents the number of moles of potassium chloride plus the number of moles of sodium chloride per cubic meter gas in the flue gases exactly upstream that channel section in which the dosing of additive takes place. For wood fuel of an presumed chlorine content of 0,027 percentage by weight, this corresponds to a sulphur addition of approx. 10-28 mg S/MJ fuel or approx. 0,18-0,49 g S/kg (total solids content TS) fuel. By calculating the specific amount of flue gas that arises at burning wood fuels according to this example, the above-mentioned dosing of additive provides an increment of sulphur in the flue gas equivalent to 24-63 mg S/m3n wet flue gas. For fuels containing a higher, in comparison with chlorine, amount of alkali being volatile at burning, for instance wood fuels, the approximate calculation of the demand of additive is relatively simple, since the absolutely predominant part of the chlorine content will be present as alkaline chloride in the flue gas when the same enters the channel section where the dosing of additive takes place. For fuels of a higher chlorine/alkali ratio, for instance certain waste fuels, a certain part of the chlorine content of the fuel will form hydrochloric acid (HCl) directly in the combustion zone, which needs to be taken into consideration at approximate calculation of a suitable quantity of additive.
The reason why a higher SO3/[KCl+NaCl] mole ratio is required in practice in comparison with the theory is that SO3, besides the alkaline chlorides, also reacts with other alkaline compounds in the flue gas and that a small share of the SO3 is reduced to SO2, where the latter in principle is inactive for reaction with the alkaline chlorides. In all events, the quantity of sulphur, which is required to attain a certain reduction of the alkaline-chloride content in flue gas, is considerably less at injection of SO3-generating additive into the flue gas in comparison with sulphur addition to the fuel.
The procedure of determining the optimum flow of SO3 additive in a given application is as follows. The additive is dosed proportionally to the fuel flow according to the formula (4) below;
Additive flow [g sulphur/hour]=fuel flow [kg TS/hour]xc3x97Kxe2x80x83xe2x80x83(4) 
The formula (4) calculates which flow of sulphur being required in order to achieve a certain reduction of chloride. When the type of sulphur compound that is used for generating the SO3 gas is known, subsequently the flow of the sulphate compound can easily be calculated. Examples concerning ammonium sulphate are shown below. The fuel flow can be calculated on the basis of the current, produced thermal effect and the total efficiency of the boiler, or measured directly by established methods. K is a parameter that varies with the chlorine content, sulphur content and alkaline content of the fuel, as well as factors specific to the plant. For instance, the parameter K may amount to ≈0,28 g sulphur/kg (TS) fuel at burning wood fuels. At such an addition of additive, a reduction of 70-80% of the alkaline-chloride content in the flue gas can be expected, which is sufficient to slow down the corrosion rate on the superheater radically. If ammonium sulphate is used as an SO3-generating additive, the flow mentioned above equals approx. 1,13 gram ammonium sulphate/kg (TS) fuel or 157 mg ammonium sulphate/m3n flue gas.
Representative indicative values of the parameter K for other solid fuels are shown below.
It should be observed that all fuel types are most often inhomogeneous. Large variations may be present both in regards of chlorine content as well as other characteristics, which implies that the optimum additive flow may need adjustment in each individual application. The adjustment may be effected on one single occasion or intermittently in those cases when a fuel of a relatively homogeneous quality is used. In cases when the quality of the fuel varies to a higher degree (when, for instance, waste fuels or a mixture of different types of bio fuels are used), continuous measurement in the flue gas should be made, at the same time as the dosing of additive is adjusted. The methods available to campaign-wise trim an optimum additive flow, or more precisely to tune the parameter K according to the relation (5), is shown below.
The necessary additive flow in order to achieve the desired reduction of the alkaline-chloride content in the flue gas may be examined with the following methods:
1) Direct in-situ measurement of the alkaline-chloride content in the flue gas in the area of the first superheater, preferably by means of the measurement method being described in SE 9903656-8. The adjustment of the additive flow is not limited by the response time of the measurement, but rather by the resetting time for the respective flow level.
2) Campaign measurement of composition and growth of the ash deposit which is formed on a temperature-controlled probe being exposed to flue gas in connection with the superheater. Such a campaign measurement does, however, usually demand 3-12 hours in order to obtain useful results.
3) Corrosion measurement with a temperature-controlled probe being exposed to flue gas in connection with the superheater. Such a measurement does usually demand 1-12 weeks in order to obtain useful results. Therefore, this is in practice very time-consuming and impractical.
4) On-line measurement of superheater corrosion using modern probe methods based on electrochemistry. The method has, however, a response time of approx. 1-12 hours in of response time subsequent to a gradual alteration of the flue-gas chemistry before reliable measurement results are obtained.
5) Measurement of HCl (the reaction product in the sulphation reaction [1]), either in the area around the first superheater or later on in the flue-gas zone. When the HCl content no longer increases with increased dosing of additive, it may be presumed that in principle all available alkaline chloride in the flue gas has been sulphated.
The above-mentioned methods 1-5 (preferably method 1) may be used to check the quantity of additive that is required to obtain the desired result. However, by said methods it is difficult to check that overdosage does not occur, with an increased risk of negative and expensive side effects. Such side effects may consist of corrosion and deposit formation on the low-temperature heat exchangers of the plant. In order to check the tendency of overdosage, the following methods may be applied:
6) SO3 measurement in the flue gas downstream the area where the additive is dosed, by means of methods known to those skilled in the art.
7) Measurement of the acid dew point of the flue gas by means of commercially available measurement equipment.
8) Measurement of corrosion and deposit formation using a probe, in principle according to methods 3) or 4), but in this case in the low-temperature section of the plant, where the flue gases have assumed a temperature of about 100-200xc2x0 C.
Dosing Equipment
An important factor in order to achieve the intended reduction of alkaline chloride is that the SO3-generating additive is injected into the flue gas in such a way that a good mixing of SO3 and the metal chlorides of the flue gas is achieved before the gas mixture hits the first superheater. More precisely, said dosing needs to be performed in such a way that the concentration ratio between SO3 and metal chloride is almost constant over a cross-section of the flue-gas channel before the flue gas hits the first superheater. If the injection should be effected in such a way that, for instance, the SO3 additive does not penetrate into the centre of the flue-gas channel, but is distributed to the non-central areas of the channel, chlorine corrosion will still be take place on the central portions of the superheater, while sulphur-related corrosion may appear on the parts of the superheater which are exposed to relatively high contents of SO3. To achieve an optimum intermixing of the additive in the flue gas is in practice made difficult by the fact that the flue-gas flow and the concentration of alkaline chlorides, respectively, is practically never entirely evenly distributed spatially across the cross-section of the flue-gas channel into which the additive is introduced. Depending on the design and the operation of the combustion plant, marked channelling may be found in the flue gas between the combustion zone and the superheaters.
In order to achieve the best possible spreading and distribution of the additive across the cross-section of the flue gas, wall-mounted nozzles or transverse lances can advantageously be used. A third alternative is to utilise some form of mixing device being located between the injection point and the superheater. Said mixing device may in practice consist of one or more cyclones, which is standard equipment in the commonly used boiler technique CFB (Circulating Fluidized Bed)
In order to attain the above-mentioned requirement of a homogenous concentration distribution of SO3 across the cross-section of the flue gas, the flow profile of the flue gas as well as the concentration profile of metal chlorides need to be mapped. This, in order to ascertain how the additive flow should be distributed over the same cross-section. It can be assumed that the alkaline chloride concentration accompanies some fuel specific component in the flue gas, for instance carbon dioxide. Mapping of the gas flow and carbon dioxide content across the cross-section of the flue gas can be carried out by measurement in full scale by methods known to those skilled in the art, by modelling in physical scale models or by mathematical modelling (so-called CFD). When the spatial flow distribution of alkaline chloride is mapped, a suitable dosing device is dimensioned and adjusted according to the above (nozzles or lance), so that the additive is added in proportion to the local chloride flow in each part of the cross-section of the flue gas. The result may be controlled by mapping the concentration distribution of a tracer component in the cross-section of the flue gas downstream the injection point, which component is added in the SO3 additive and which is possible to measure. If for instance an ammonium-sulphate solution is used, ammonia (which is formed when the ammonium sulphate is heated) can be used as a tracer component.
The number, dimensioning, and location of the nozzles should be carefully chosen in each individual application. Normally, at least two nozzles should be used. Liquid additives have in this regard obvious advantages over additives of gaseous phase or solid phase by the forming of drops in the nozzles, which easily may be ejected several meters counter-flow into the flue gas. If for instance a 25% ammonium sulphate-solution is injected, the liquid flow will be so small in relation to the flue-gas flow that it may be necessary to add dilution water to the sulphate solution before the solution is pumped into the nozzles. This, in order to achieve a sufficient drop size and penetration depth in the flue-gas channel. In the most common commercial nozzles, compressed air is also added in order to control drop size, distribution angle and ejection length.